1. Field of the Invention
The present invention relates to a rubber composition having excellent processability and improved dispersion for carbon black.
2. Description of the Prior Art
In recent years, with trends towards higher performance of automobiles, requirements with respect to safety, high-speed performance, fuel consumption and the like are becoming more and more severe.
To satisfy these requirements from the position of carbon black to be added to solid rubber to reinforce it, thereby obtaining a tire having both good gripping property and anti-wet-skid property while maintaining good wear resistance that can withstand high-speed stable running, it is reportedly effective to decrease the primary particle diameter and increase the specific surface area of carbon black.
However, too small a primary particle diameter of carbon black decreases the processability so that incorporation of the carbon black into rubber becomes difficult. Forced dispersion of such carbon black in rubber by for example prolonged kneading time still cannot satisfy the property requirements due to decreased dispersibility.
Use of additives to improve dispersibility of carbon black has been attempted. For example, Japanese Patent Application Laid-open No. 20579/1992 proposes addition as a dispersing agent for carbon black of an ester of an unsaturated aliphatic acid having 8-24 carbon atoms or that of an unsaturated alcohol having 8-24 carbon atoms. However, these additives, having small molecular weights, have the drawback of decreasing the mechanical strength of the resulting rubber compositions.
Journal of Japan Rheology Association, Vol. 20, 18 (1992) describes that polymers having terminal carboxyl group are effective for dispersing pigments. The polymers with terminal carboxyl group, their main chain being a polyester (polycaprolactone), have the drawback of very poor compatibility when used as a dispersing agent for carbon black for rubber, thereby being inferior in processability and decreasing mechanical properties, as shown in Comparative Example 5 to be described later herein.
Japanese Patent Application Laid-open Nos. 105844/1974 and 89446/1975 describe a processing of a conjugated diene-based low-molecular-weight polymer comprising incorporating carbon black into the conjugated diene-based low-molecular-weight polymer having at least one functional group. In this process, the functional group possessed by the conjugated diene-based low-molecular-weight polymer has been introduced thereinto to act as a crosslinking point when this polymer is to be cured by crosslinking. Thus, these literature have no technical thought of improving the dispersibility of carbon black by the use of a conjugated diene-based low-molecular-weight polymer having functional group.
Japanese Patent Application Laid-open No. 124639/1985 discloses a composition having good adhesion obtained by adding to a solid rubber a diene-based liquid rubber having at least 3 hydroxyl groups in the molecule thereof. On this occasion, there is employed a process for producing the diene-based liquid rubber having hydroxyl groups which comprises introducing maleic anhydride into a diene-based liquid rubber and then reacting the modified liquid rubber with an amino alcohol to introduce hydroxyl groups. This process, however, permits maleic anhydride to bond uniformly to the middle part and both ends of the diene-based liquid rubber, thereby being unable to control the bonding positions for hydroxyl groups in the main chain of the diene-based liquid rubber. Besides, the literature mentions nothing about the effect of improving dispersion of carbon black as produced by addition of the diene-based liquid rubber having hydroxyl groups.
Japanese Patent Application Laid-open Nos. 127952/1977, 51146/1983 and 61135/1983 describe that rubber compositions obtained by adding a functional group-containing polymer to a solid rubber is effective in improving adhesion with materials of different nature. However, any one of the rubber compositions must contain as an essential component an organometal compound. Besides, none of the literature mentions improvement by the functional group-containing polymer in dispersibility of carbon black.
Japanese Patent Application Laid-open No. 133436/1980 describes that a rubber composition was obtained by adding to a vulcanized rubber powder a liquid polyisoprene rubber modified by addition reaction of maleic anhydride or its derivatives. In this composition, however, the modified liquid polyisoprene rubber is to act as a binder and carbon black is not an essential component.